Theoretical study on the X-Ray absorption at the sulphur K-edge in gold nanoparticles protected by thiolates

A time-dependent density functional theory method has been employed to study the XANES sulfur K-edge spectra in a series of gold methylthiolate clusters, taken as molecular models for gold nanoparticles protected by dodecanethiol. The calculated spectra of all the clusters here considered show two intense maxima, separated by 2-3 eV, preceded by a weak feature. This latter is sensitive to the nature of the S-Au coordination geometry. On the basis of the calculations, we have proposed a revised interpretation of the experimental spectrum: the intense experimental maximum is actually contributed by the two intense maxima found in the calculations, which are not resolved in the measurement. The weak experimental feature at low energy suggests that in the real gold nanoparticle the thiolate acts as a bidentate ligand in the S-Au coordination